In the field of electroplating an article intended to be plated is placed in an electrolytic bath. The plating material, such as gold, zinc, nickel, silver or other material or a combination of materials intended to be plated onto the article, is dissolved in or is otherwise conveyed into an electrolyte which forms the electroplating bath. Often the plating material is derived from one or more anodes positioned in the bath. The anode is coupled to a power supply and the article intended to be plated also is coupled to the power supply and serves as the cathode in the electrolytic plating system. By applying an electrical potential difference/voltage between the anode and cathode a current flows therebetween through the electrolyte and the plating material migrates to the article intended to be plated. The amount of plating material actually plated onto the cathode/article intended to be plated is a function of the applied voltage, current flow through the electrolyte and current density at the article intended to be plated. When the current density varies at different parts of a particular cathode/article, the degree of plating there also will tend to vary correspondingly. Also, if current is switched on and off, laminar plating may occur, which often is undesirable. Laminar plating is a formation of layers rather than a homogeneous plating.
Current tYpicallY is a function of applied voltage and the impedance, as is well known. The impedance typically is a function of the efficiency of the electrodes themselves (including electrochemical equivalents thereof), of the impedance characteristics of the plating bath, and of the spacing of the electrodes in the bath.
It would be desirable to maintain a controlled and uniform current density at the cathode/article intended to be plated in order to achieve a desired controlled, uniform plating thereof.
One type of alloy plating uses the anodes themselves to contribute to the composition of the bath. In such case plural plating materials are applied to the article intended to be plated as an alloy or mixture of such plating materials. The problems encountered with non-uniform current density are further complicated by the additional factor that the concentration of plating material ingredients may vary with time, voltage, plating that forms on a particular anode or cathode, polarization of an anode, efficiency of the anode and/or cathode, current density, etc. For example, in one alloy plating system with respect to which the invention will be described in detail below, there may be multiple anodes, each of which contributes a separate ingredient into the electrolytic plating bath to form the plating material alloy. The concentration of one plating material relative to the other or others must be maintained in the electrolytic plating bath to obtain the desired constituency or relative concentrations of plating materials on the article intended to be plated. For example, to obtain a plated coating of an alloy of zinc and nickel on a metal article, say of a constituency of 88% zinc 12% nickel, it is necessary that the relative amounts of such zinc and nickel ingredients in the electrolytic bath be at approximately a 62/38 ratio or some other known ratio that is altered according to some function, such as a function of the ionic nature of the respective ingredients that is representative of the tendency of such material to plate onto the cathode/article, the operative efficiency of the anode(s) and/or cathode, etc., as is known. For example, it is known that one material may plate more readily than another due to the fact that there are more free electrons available in one than in the other or there is some difference between the ions as they travel through the electrolytic plating bath from the anode to the cathode.
However, as the concentration of one ingredient relative to the other (others) in the electrolytic plating bath changes, regardless of whether such concentration change is due to anode dissolving or some other reason, the ratio of those ingredients in the finished plating coating will change, which may cause a variation from specifications for the finished plated article. This, of course, is undesirable.
To alter the concentration relationship of the electrolytic plating bath to bring it back to the desired specification, it is necessary to increase the amount of one ingredient relative to the other. Such increase sometimes is brought about by changing, say increasing, the current/potential difference between the anode that is supplying such ingredient and the cathode. Such a change in current, though may cause a change in current density at the cathode if a corresponding adjustment in the current/potential difference between the other anode and the cathode is not made, which changes the uniform plating thereat and also can cause a difference in the amount of one ingredient that is plated onto the cathode/article relative to the amount of the other ingredient that is plated onto the cathode/article. Such a change in current also causes a potential difference between the anode whose voltage has been changed by increasing current there and the other anode which is supplying the other ingredient to the plating bath, thus possibly causing undesirable plating of the first ingredient onto the other anode, which in turn can cause an undesirable shift in operation, anode efficiency, and/or concentration in the electrolytic plating bath.
In other types of electroplating ingredients are directly added to the bath to obtain plating material therein. The invention is useful in such plating techniques.
Another type of electrolytic processing is known as electrolytic polishing or simply electropolishing. In electropolishing high points or roughness causing flaws in the surface of a material intended to be polished are removed to improve smoothness of the surface. In electropolishing the part intended to be polished is placed in an electrolytic bath and serves as one electrode of the electropolishing system. A second electrode also is placed in the bath. A potential difference of prescribed polarity is applied between such electrodes so that current flows from the part toward the other electrode. Due to the direction of current flow, the part is referred to as the anode, and the other electrode is referred to as the cathode. Since the high points on the part tend to concentrate current or at least tend to experience higher current density than the already smoother surface portions, material at the high points tends to be removed with the current flow, thus effecting polishing. The various problems encountered in electroplating also can detrimentally affect electropolishing.
The present invention helps solve the above problems and disadvantages encountered in prior electroprocessing systems and methods.
Another problem encountered in the past has been the tendency of some electroplating baths to grow or to increase in the amount of a particular ingredient therein or ratio of that ingredient compared to another. Sometimes additives have been added to the bath to control such growth, and sometimes inert (non-dissolving) electrodes (anodes) have been used to control such growth. The present invention described below may be used to control power to such control electrodes, i.e., the inert electrodes, therefore controlling the composition of the bath.
Further, in the past electrodes formed of an alloy material have been used to provide the desired concentration of alloy ingredients to an electroplating bath. One example is an alloy of tin and lead. Using the present invention, though, separate electrodes, one of tin and the other of lead, can be controlled to provide the desired concentration of ingredients to the plating bath.
As used herein anode, cathode or electrode may refer to one anode, cathode or electrode or to a group of anodes, group of cathodes or group of electrodes, as will be evident to those having ordinary skill in the art.